980 ti or titan x for 1080p/4k

Outta the box, the Titan X at 1080p / 1440p but not by enough of margin to warrant the price increase. As for RAM, 4 GB is enough for those resolutions. OTOH, the 980 Ti gets up to 31% overclocks, the Titan X, 11% so the 980 Ti will rule when OC'd.

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At 4k, frankly no single card is enough for 4k ... I don't see 4k being attractive (60 fps or better, even in SLI until late 2016 at the earliest. And I wouldn't buy a 4k monitor now anyway until Display Port 1.3 and 120 Hz arrives

 
Factory overclocked GTX 980 Ti's with custom cooling will always be faster than a TitanX. The reference cooler and reference components common to all TitanX's ensures that their Turbo Boost clocks will be modest in comparison. Pick a custom MSI GTX 980 Ti Gaming with the Twin Frozr cooler, or one of the very nice models with all-in-one liquid cooling, and the TitanX just becomes an expensive and lower performing marketing gimmick for people where "budget is not a problem".